Many recent studies have demonstrated the efficacy of interstitial ablative approaches for the treatment of malignant and benign tumors, including chemical ablation, cryoablation, and thermal ablation using energy sources like radiofrequency, laser, microwave, or focused ultrasound. Despite these promising results, current systems remain highly dependent on operator skill, and cannot treat many tumors because there is insufficient control of the size and shape of the zone of necrosis, and no control over ablator trajectory or energy directivity within tissue. Remedying this problem requires advances in end-effector design, precise conformability of the ablation volume and shape created by the ablator device to the desired target location, and real-time monitoring of the zone of necrosis to ensure complete treatment. While intra-operative ultrasound imaging has been shown to be a practical and affordable in targeting, simultaneous manual handling of the B-mode ultrasound (US) probe and the ablator device is a challenging task that is prone to significant errors in the hands of even the most experienced physicians. Further, B-mode US imaging is not suitable for accurate real time monitoring of the ablation region to enable feedback control of the size and shape of the lesion. Tissue deformation and target motion make it extremely difficult to place the ablator device into the target. Irregularly shaped target volumes typically require multiple insertions and several overlapping thermal lesions, which are even more challenging to accomplish in a precise and timely manner without causing excessive damage to surrounding normal tissues. In answer to these problems, embodiments described herein provide an innovative method for combined thermal monitoring and accurate tracking and registration with respect to spatially-registered intaoperative US volume. A system incorporating this three-dimensional ultrasound (3DUS) with a high intensity ultrasound ablation tool, Ultrasound Interstitial Thermal Therapy (USITT), capable of actively shaping ablation, under real-time monitoring from registered thermal imaging. Our interstitial and intracavitary high-power ultrasound applicators have demonstrated controllable and penetrating dynamically shaped heating patterns (dynamic adjustment in length, angle, radial penetration), providing an ideal mechanism for precision conformable thermal surgery. This controllability and penetration has the potential to provide a significant improvement over existing radiofrequency (RF) and microwave (MW) technology used for minimally invasive thermal ablation of liver tumors, which are limited to fixed (generally spherical) or unpredictable ablation profiles and comparatively low therapy penetration depth. To date, extensive evaluation of this minimally invasive technology has been limited mostly to in vivo canine prostates and other moderately perfused tissues. In contrast, embodiments of the present invention include a true closed-loop system for placement, guidance, and percutaneous delivery of conformal ultrasound ablative therapy, with on-line monitoring of treatment using thermal imaging.
Current monitoring approaches often result in either positive margins or excessively large ablation zones in order to achieve negative margins. Some ablative devices employ integrated thermistors or thermocouples for temperature monitoring. However, these temperature readings only provide a crude estimate of the true zone of ablation. Non-invasive monitoring options include US, magnetic resonance, CT, and X-ray fluoroscopy.
Ultrasound Imaging. Conventional ultrasonographic appearance of ablated tumors only reveals hyperechoic areas due to microbubbles and outgasing. According to Kolen et al. [Kolen-2003], high-intensity focused ultrasound (“HIFU”) experiments show that B-mode imaging is generally inadequate. The size and shape of the hyperechoic region in the B-mode does not necessarily correspond to the damage seen on the gross-pathology pictures.
MRI Imaging. Magnetic resonance imaging can monitor temperature changes (MR thermometry), but is expensive, limited in availability, difficult to use intraoperatively, not real time, and lacks implementation flexibility. MR thermometry provides low frame rates and requires specific MRI-compatible equipment. [Graham-1999].
CT and X-ray. These technologies are capable of measuring soft tissue mass density changes. Salas et al. [Salas-2004] introduced a new method that should aid all thermal ablative techniques. The method requires an X-ray imaging system with a digital detector. During ablation, periodic X-ray exposures are taken and subtracted from a baseline pre-ablation regional X-ray mask. Successive subtracted images show the propagation of the change in density, which is indicative of coagulation.
Ultrasound Elasticity Imaging (USEI). This imaging modality has emerged as a useful augmentation to conventional US imaging. USEI has been made possible by two discoveries: (1) different tissues may have significant differences in mechanical properties and (2) the information encoded in the coherent scattering (a.k.a. speckles) is sufficient to calculate these differences following a mechanical stimulus [Ophir-1991]. An array of parameters, such as velocity of vibration, displacement, strain, strain rate, velocity of wave propagation and elastic modulus, have been successfully estimated [Konofagou-2004, Greenleaf-2003], which makes it possible to differentiate stiffer tissue masses, such as tumors [Hall-2002, Lyshchik-2005], or ablated lesions [Varghese-2004].
Ultrasound Thermal Imaging. Temperature estimation algorithms using ultrasound are very similar to strain estimation; both attempt to solve similar time-delay estimation problems. FIG. 1 represents the main concept of heat-induced echo-strain. Prior to target zone heating: the overlay represents the same speed of sound (i.e. same temperature). During ablation: the overlay represents the hot area in the middle with higher speed of sound. By tracking shifts in the backscattered RF signals, heat-induced strain can be generated, as shown in the strain profile, where low strain corresponds to constant temperature and high strain to higher temperature. The induced shift in the RF backscattered signals is related to both changes in speed of sound and thermal expansion, which can be calibrated to correspond to temperature. Two-dimensional temperature estimation using pulse-echo diagnostic ultrasound has been previously described by Ebbini et al. [Seip-1995, Simon-1997], by Damianou et al. [Maass-1996] and by Varghese et al. [Varghese-2002]. Measurement models in both time and frequency domains have been proposed in the literature [Amini-2005, Anand-2007, Maleke-2008].